1. Field of Invention
This invention relates to airships intended to be stationed at high altitudes for extended periods of time.
2. Prior Art
High-altitude, long-endurance airships have been proposed as alternatives to satellites as less expensive platforms for surveillance and communications applications. Charged with a lifting gas such as hydrogen or helium, an airship can rise high above the earth, providing a wide area of coverage. Typical altitudes are above the tropopause, at altitudes of the order of 20-22 km above the earth, and above cloud cover. Automatic control systems linked to global positioning satellites allow uninhabited operation.
A major challenge is to keep an airship more or less over the same spot on earth. Absent some means of propulsion, winds aloft would otherwise cause the airship to drift from its intended location. To maintain its station, the airship must constantly fly into the wind at wind velocity.
During daylight hours, the airship has access to solar power. That power can be captured by solar cells and used to drive propellers with electric motors to propel the airship. However, powering the airship at night presents a serious problem.
Various schemes have been proposed for storing solar energy generated during daylight hours for use at night. They include such energy-storage devices as batteries and regenerative fuel cells. However, the buoyancy of an airship is limited by the weight of the air it displaces. At proposed operating altitudes, the air density is low and most of the lifting capacity of the airship is used in supporting the weight of airship itself. No energy storage system yet devised is light enough to store all the power needed for station-keeping at night.